(i) Field of the Invention
This invention relates to a coupling mechanism particularly suitable for use as a starter, employing a centrifugal lock-up clutch which permits a broad freedom in setting variations of torque capacity for power transmission, in combination with a fluid coupling device such as torque converter, fluid coupling or the like.
(ii) Description of The Prior Art
In transmissions of motor vehicles, it has been known to use a direct coupling clutch (or a lock-up clutch) in a coupling mechanism employing a fluid coupling, for the purpose of enhancing the efficiency of power transmission. The lock-up clutches are generally classified into a piston type and a centrifugal type, of which the piston type has drawbacks in that it incurs a higher cost and needs a complicate control mechanism. With a view to eliminating these drawbacks, the present inventors previously proposed in Japanese Patent Application No. 58-205299 a starter which employs centrifugally rotatable cam weights for a centrifugal clutch, thereby permitting a large increase of clutch torque capacity in a low speed range and improving durability of the lock-up clutch. However, the torque capacity of the centrifugal clutch of our previous proposal depends on the rotational speed of the turbine runner, so that there arises a problem that the durability of the lock-up clutch is deteriorated in a high-load low-speed operation, for example, in a long hill-climbing operation on steep slopes due to the heat which is generated by slipping of the lock-up clutch. Besides, the torque capacity depends on the rotational speed of the turbine runner alone, so that it is necessary to use heavier friction shoes and cam weights in order to secure large torque capacity (for application to a high power engine), and to sacrifice compactness in order to ensure strength of component parts.